This invention relates to a glass-ceramic for magnetic disks having a crystal structure suitable for use as a material for a magnetic disk substrate and having improved surface characteristics after polishing and a method for manufacturing the same.
There has been a rapid progress in developments in magnetic diks demand for which has recently increased remarkably as external media of large scale computers and personal computers. Characteristics required generally for a magnetic disk substrate are as follows:
(1) In start/stop (CSS) characteristics of a magnetic disk, the magnetic disk should have not a completely smooth surface but a surface whose surface roughness (Ra) is controlled to a range from 15 .ANG. to 50 .ANG. in order to prevent adsorption between the head and the disk and damage to the head and media. PA0 (2) Since the amount of lifting of the head tends to decrease to the order of 0.1 .mu.m to 0.05 .mu.m owing to improvement of recording density of the magnetic disk, the surface of the disk must be flat and relatively smooth. PA0 (3) The material of the magnetic disk substrate must not include anisotropic crystal or other defects and its structure must be dense, homogenious and fine. PA0 (4) The material must have sufficient mechanical strength and hardness for standing a high speed rotation and-contact with the head. PA0 (5) The material must not contain Na.sub.2 O ingredient in principle because the Na.sub.2 O ingredient causes dispersion of sodium ion during the film forming process with resulting deterioration in characteristics of the film. PA0 (6) The material must have chemical durability against rinsing and etching with various chemicals. PA0 (1) Polishing is made after chemical tempering and so the tempered layer tends to cause instability in thinning the disk substrate. PA0 (2) For improving the start/stop (CSS) characteristics, the substrate must be subjected to processings for improving its mechanical and chemical textures. This prevents mass production of the product at a competitive cost. PA0 (3) Since the Na.sub.2 O ingredient is included as an essential ingredient in the glass, the film forming characteristic of the glass is deteriorated with the result that a surface coating processing becomes necessary. It has recently been put into practice in the field of chemically tempered glass and crystallized glass-ceramic to make the surface of the glass rough during the polishing process for overcoming the problem of the texture processing for improving the start/stop (CSS) characteristics. This technique however is not sufficient either for realizing a stable mass production of the product at a competitive cost.
Aluminum alloy has been conventionally used as a material of magnetic disk substrate. The aluminum alloy substrate, however, tends to produce a substrate surface having projections or spot-like projections and depressions during the polishing process due to defects inherent in the material. As a result, the aluminum alloy substrate is not sufficient in flatness and surface roughness and cannot cope with the recent requirement for high density recording necessitated by increase in the amount of information.
As a material for overcoming this problem of the aluminum alloy substrate, known in the art is a glass substrate for magnetic disks made of chemically tempered glass. This glass substrate, however, has the following disadvantages:
Aside from the aluminum alloy substrate and chemically tempered glass substrate, known also in the art are glass-ceramics which satisfy some of the above described requirements. For example, the SiO.sub.2 -Al.sub.2 O.sub.3 Li.sub.2 O system crystallized glass-ceramic described in the Japanese Patent Application Laid-open No. Sho 60-229234 contains beta-quartz solid solution or beta-spodumene solid solution and has a grain diameter of crystal ranging from about 0.1 .mu.m to about 1.0 .mu.m. For another example, the SiO.sub.2 -Li.sub.2 O system glass-ceramic described in the Japanese Patent Application Laid-open No. Sho 62-72547 contains lithium disilicate and lithium metasilicate as a main crystalline phase. The lithium disilicate phase has a plate-like crystal having a grain diameter of crystal in the range from about 0.3 .mu.m to about 1.5 .mu.m and the lithium metasilicate phase has a granular crystal having a grain diameter of crystal in the range from about 0.3 .mu.m to about 0.5 .mu.m. These glass-ceramics, however, require the texture processings for making the surface of the glass-ceramics rough after the polishing process for improving the start/stop (CSS) characteristics which are essential for the material of magnetic disk substrate.
For overcoming these problems, the Japanese Patent Application Laid-open No. Hei 2-247821 proposes a glass-ceramic containing a crystal phase having a texture of sheet-like or chain-like silicate. The anisotropic crystal of this glass-ceramic having an acicular or rod-shaped structure causes significant holes and pits with the result that a sufficient surface roughness cannot be obtained. Further, since the crystal of this glass-ceramic contains Na.sub.2 O ingredient, there occurs dispersion of sodium ion in the film foaming process and this adversely affects the characteristics of the film.
It is, therefore, an object of the invention to provide a crystallized glass-ceramic for magnetic disks which has eliminated the disadvantages of the prior art products and has excellent surface characteristics after polishing by controlling the crystal structure and crystal grain of a crystal produced. It is also an object of the invention to provide a method for manufacturing such glass-ceramic.